Add a small JS API.

Adds a global variable to browserid.js (django_browserid) that has 3
useful methods: login and logout for manually triggering BrowserID,
and isUserAuthenticated for checking if the current user is 
authenticated.

We still need to add some automated testing for the JS, but in the
meantime I've tested this on my test playdoh site and it seemed to
work fine. I'll also be using this on Flicks very soon (guess what
motivated me to add these? :P).